Galileo is an ambitious, multiple-vehicle planetary mission. It has two major interlocking elements: a probe to be placed in the atmosphere of Jupiter and an orbiter to explore Jupiter, its satellites, and its magnetosphere. By using individual satellite flybys to alter its orbit, the Galileo spacecraft can carry out a satellite “tour” consisting of flybys of the Galilean satellites at different geometries and a deep penetration into the magnetosphere in the unexplored region of space behind Jupiter. Both an atmospheric probe for the planet and a long-lived orbiter to study the satellites and magnetosphere are logical successors to Voyager. In 1974, three years before Voyager launch, the Space Science Board of the National Academy of Sciences was already emphasizing the scientific advantages of both of these approaches. In suggesting goals for 1975-1985, the Board wrote, “We recommend that a significant effort in the NASA planetary program over the next decade be devoted toward the outer solar system. Jupiter is the primary object of outer solar system exploration.” Looking at specific mission goals, the Board recommended that “the primary objectives in the exploration of Jupiter and its satellites for the period 1975-1985 in order of importance are (1) determination of the chemical composition and physical state of its atmosphere, (2) the chemical composition and physical state of the satellites, and (3) the topology and behavior of the magnetic field and the energetic particle fluxes. In order to carry out this program, it will be necessary to utilize orbiting spacecraft and probe-delivering spacecraft.” In the same period NASA carried out studies of two possible orbiter and probe missions. Working through the Ames Research Center, a scientific panel chaired by James Van Allen explored the adaptation of the Pioneer 10 and 11 spinning spacecraft to carry a probe to Jupiter and to carry out an orbiter mission emphasizing magnetospheric studies. William B. Hubbard of the University of Arizona chaired a JPL-based panel investigating the use of a Mariner-class fully stabilized spacecraft similar to Voyager to carry out a satellite-oriented orbiter mission. In 1976 these concepts were combined in a study, again chaired by Dr. Van Allen, of a Voyager-type orbiter with probe-carrying capability. This mission concept was given the name JOP, for Jupiter Orbiter Probe, and lead responsibility was assigned by NASA to JPL, with Ames carrying out the design of the probe.